The electrical properties of the earth can be measured using metallic or non-polarizing electrodes, which make direct galvanic contact with the earth, or by inducing an electromagnetic field into the earth. The invention proposed utilizes electrodes to make direct galvanic contact (DC resistivity method) with the earth and does not enable electromagnetic induction measurements. The invention proposed does not make the geoelectrical measurement, rather it serves as the tool which couples the earth to an industry standard acquisition instrument.
The propagation of electrical current into the earth is governed by the earth material (i.e. consolidated or unconsolidated rock) and the electrolyte, which is the fluid occupying the pore space, within the earth material. The rock or sediment type has characteristic electrical resistivities (resistance to electrical current propagation). Furthermore, if the volume of earth that is measured contains a conductive electrolyte within the pore space of the measured volume, the geoelectric response will be less electrically resistive.
This invention utilizes electrodes which are in contact with the earth. The DC resistivity method uses electrodes that are configured in certain geometric configurations to measure the resistivity of a volume of the earth. Therefore the geoelectrical measurement is indicative of the geoelectrical properties of the volume of earth measured. Such a measurement is termed the apparent resistivity as it is the effective resistivity of the volume of earth determined by this geometrical arrangement of the electrodes.
The DC resistivity method has been used since the early part of the 20th century. DC resistivity can be used with surface electrodes, electrodes down an open borehole, or with permanently emplaced downhole electrodes. This invention is a specific type of the latter.